In semiconductor fabrication processes, the resolution of a photoresist pattern begins to blur at about 45 nanometer (nm) half pitch. To continue to use fabrication equipment purchased for larger technology nodes, multi-exposure methods have been developed.
Multiple exposure or multi-patterning technology (MPT) involves forming patterns on a single layer of a substrate using two or more different masks in succession. If only two masks are used for patterning a layer, the technique is referred to as double exposure. One form of double exposure is referred to as double patterning technology (DPT). In DPT, first and second masks are used sequentially to pattern the same layer. As long as the patterns within each mask comply with the relevant minimum separation distances for the technology node, the combination of patterns formed using both masks may include smaller spacings than the minimum separation distance.
DPT is a layout splitting method analogous to a two-color problem for layout splitting in graph theory. For example, if two masks are to be used (double patterning, DPT), it is customary to refer to the patterns as being assigned one of two “color types”, where the color corresponds to a photomask assignment.
If there is any mask alignment error, the patterns formed by the second photomask are shifted relative to the patterns formed by the first photomask. A pattern formed by the second photomask can be closer to, or further from, a given pattern formed by the first photomask, causing the parasitic resistance-capacitance (RC) coupling between the patterns to differ from the couplings computed by RC extraction, which are used for timing analysis.